Hmmmm tricky, catch is that necromancer only summons undead and leeches life and mess around with poison. Witches on other hand summon demon, mess with supernatural, mess with diseases (also they can cure diseases too) and technically I think that Necromancy is similar to witchcraft cause and witches and necromancers mess in witchcraft only different branches.
https://en.wikipedia.org/wiki/WitchcraftI had not read whole page but you are free to look
